Doktorsavhandling, 2007

Synthesis and Self-Assembly of Functional Molecules:
Systems Toward Molecular Electronics
Daniel Nilsson
Organic Chemistry, Department of Chemical and Biological Engineering
Chalmers University of Technology
Abstract
In this Thesis, different molecules with possible use in molecular electronics are reported. The focus is on the synthesis of the molecules and their use as adsorbates in self-assembled monolayers (SAMs). For molecular electronics it is often desirable to have a high conductivity, and therefore the oligo(phenyleneethynylene) molecular wire framework is extensively explored in this work.
Three molecular wires with different lateral bulk (benzene, naphthalene, and anthracene) were synthesized and evaluated. It was found that the close packing of the SAM-adsorbates is influenced by the varying bulk. The tilt angle of the SAM is smallest for the benzene adsorbate, larger for the naphthalene one, and largest for the anthracene adsorbate.
In order to form a SAM the molecular wire must have a moiety that can interact with the substrate surface. The TMS-ethynylene group is compared to the TMS-vinylene and TMS-phenylene groups as an anchor group for self-assembly on gold. The TMS-ethynylene moiety forms the best SAM of these three. While the TMS-vinylene group attaches the adsorbate to the substrate, it does not support formation of a commensurate SAM. The TMS-phenylene group does not bind to a gold surface.
By combining a molecular wire with a porphyrin moiety a more complex adsorbate is produced. The redox properties of the porphyrin are relevant for molecular electronics. Three different anchor groups are used to attach porphyrin-molecular wire adsorbates onto gold, and the formed SAMs are analysed. All three systems produce SAMs, but the adsorbate using a thiophenolic anchor group results in the most upright and ordered SAM.
The synthesis of a Zn-Au porphyrin dimer linked together by a naphthalene OPE derivative is presented. This dimer is one of several that are used for charge recombination studies, i.e. transport of electrons via the OPE bridges.
Thiols are the most common anchor group for SAMs on gold. There is a need for alternative thiol protective groups during the synthesis of the SAM-adsorbates. Six different protective groups were evaluated with respect to the reaction conditions used for the Sonogashira cross-coupling and fluoride ion induced removal of a Si-based alkyne protective group.
In summary, all efforts have been centred on molecules and molecular arrays which purpose is to path the way towards molecular electronics.